Measurable components can be gallium arsenide (GaAs) or silicon (Si), FET or bipolar transistors, for small or large signals, encapsulated in cases or as chips, etc.
The measurements can obtain output, input or mutual characteristic curves (drain current I.sub.D versus drain voltage V.sub.D for different gate voltages V.sub.G, gate current I.sub.G versus gate voltage V.sub.G or drain current I.sub.D versus gate voltage V.sub.G for different drain voltages V.sub.D in case of FETs), all under static conditions, i.e. under current test conditions.
These types of measurement prove useful both to determine the electrical behavior of the component, and to predict its reliability. In fact they allow bias current and voltage measurements for a particular quiescent operating point and their possible variations under extreme thermal conditions.
Making these measurements in practice, however, presents some difficulties, chiefly due to the rise of spurious oscillations in the component under test. The spurious oscillations not only alter the voltage and current values, but also can cause destruction of the components.
These spurious oscillations occur principally when a high current flows through the component, since in these conditions the transconductivity value increases and hence the gain of the whole measuring circuit increases as well. Low intrinsic capacitances of the component and those between the terminals and connections to bias networks are sufficient to cause a positive reaction to a determined frequency.
It is to be added that these components, meant to operate in the microwave domain, have very high cutoff frequencies (even 30 GHz) and hence extremely high gains at very low frequencies, at which spurious oscillations can take place easily.
The measurement circuit must then be stabilized over a wide frequency range, practically starting from dc current, so as to obtain reliable measured values and avoid the risk of destroying the component. In case of FET, the destruction is generally due to the failure of the gate junction, namely a Schottky type junction, directly biased when wide amplitude oscillations are present.
Different methods are presently used in laboratories to avoid spurious oscillations in the test circuit and hence to overcome the disadvantages above. A first method uses series resonant circuits and RC circuits, placed in parallel with the output circuit, which operate as loads at the spurious oscillation frequency. This method, however, is not of immediate use, since these circuits have a frequency-selective behavior. In addition, since they are implemented with lumped elements, the previous knowledge of the frequency of the spurious oscillation is required in order to obtain the tuning of the above circuits for each type of active component.
Another method consists of inserting a stabilizing resistance in series with the input circuit, connecting it as close as possible to the component itself in order to reduce as much as possible reactive parasitic parameters. This resistance reduces the stage gain over a very wide frequency band, avoiding spurious oscillations. Yet it allows neither direct voltage measurement at the input port, nor current measurement through the output port when a considerable current amount flows through the input port. In fact, in this case voltage drop across the stabilizing resistance is considerable. Consequently, the voltage measured at the output terminal and the corresponding output current are considerable.
In a further method, the test component is surrounded by electromagnetic wave absorbent materials, such as certain graphite-loaded spongy materials. It is clear that in this case, encumbrance is considerable, especially if the number of test fixtures is high, the effectiveness is poor especially at the lowest frequencies, where the active component is more unstable